The invention relates to a composition and a method for the deposition of conductive polymers on dielectric substrates. In particular, the invention relates to a composition and a method used in a process for metalizing the surface of a dielectric substrate as well as to metalize the surface of drilled through holes and microvias in dielectric substrates commonly used in the manufacturing of printed circuit boards (PCB).
The use of electrically conductive polymers in the metallization of a dielectric polymeric resin substrate was disclosed by Hupe et al. (U.S. Pat. No. 5,194,313) for the metallization of epoxy resin substrates in PCB manufacture. The method described therein involved the oxidation of exposed surfaces of the polymeric resin substrate with an oxidizing agent, such as permanganate, followed by deposition of a conductive polymer from a catalyst solution comprising a polymerizable heterocyclic aromatic molecule and an acid. Exemplary heterocyclic aromatic molecules in the catalyst composition were pyrrole, furan, and thiophene. The heterocyclic aromatic molecules polymerize over the oxidized exposed surfaces of the polymeric resin substrate, and the deposited polypyrrole, polyfuran, or polythiophene rendered the exposed surfaces of the epoxy resin substrate electrically conductive. The conductivity achieved by the conductive polymers could be used for electrolytic copper plating on dielectric surfaces. For example, the process was used to render electrically conductive the exposed side walls of drilled through holes in a copper clad laminate for subsequent copper plating. Advantageously, the oxidation step was selective for the exposed areas of epoxy resin, i.e., the sidewalls of the drilled through holes, and did not render copper laminate catalytic to polymerization.
Jonas et al. (U.S. Pat. No. 5,403,467) disclosed poly(3,4-ethylenedioxythiophene) (PEDOT), a specific conductive polymer for use in rendering polymeric resin substrates amenable to electrolytic copper plating. As currently practiced, metallization of through holes and microvias in plastic substrates involves several steps:
Drilling, conditioning, rinsing, oxidizing, rinsing, catalyzing, rinsing, and plating.
Alternatively to PEDOT polyaniline (PAni) can be used to render the dielectric surfaces electrically conductive for subsequent electrolytic metal plating.
In EP 1 897 974 A process for the metallization of a surface of a dielectric substrate with electrolytically plated copper is disclosed using conductive polymers to form a first conductive layer on the substrate surface, like e.g. the inside surface of through-holes in PWBs.
Although conventional processes are effective for metalizing dielectric substrate surfaces, e.g. epoxy resins of the type used in PCB manufacture, compositions and processes known from the art for the deposition of conductive polymers have only a limited lifetime and stability, so that an exchange of the used compositions for fresh ones is needed in relative short intervals, e.g. five to seven days.